The present invention relates to a protection circuit for a tap-grounded leakage transformer which boosts input AC power to light gas-filled lamps such as neon or argon tubes and, more particularly, to a protection circuit for a leakage transformer having its secondary coil grounded at the midtap thereof which shuts off the input AC power upon detection of an abnormality such as the grounding of a load or wiring, disconnection of load wiring, or cracking of the lamp envelope.
FIG. 1 depicts a conventional leakage transformer 11 of this kind and a ground fault protection circuit 10. A primary coil 12 of the leakage transformer 11 has its both ends connected via a switch 13 to input terminals 14 and 15, and two secondary coils 16 and 17 have their inner ends C interconnected each other and also connected to an earth terminal 18 of a transformer case 36, that is, to the case 36. The earth terminal is grounded, and outer ends of the second coils 16 and 17 are connected to output terminals 21 and 22, between which there are connected gas-filled lamps 23 such as neon or argon tubes. AC power, for example, commercial power is applied across the input terminals 14 and 15 and is boosted by the transformer 11 to light the lamps 23.
The protection circuit 10 which cuts off the input AC power upon detecting a ground fault which is caused, for example, by accidental contact of the gas-filled lamp 23 or its wiring with the case 36. Provided adjacent the secondary coils 16 and 17 are tertiary coils 25 and 26 which are magnetically coupled therewith and form part of the protection circuit 10. Normally the secondary coils 16 and 17 are wound around the tertiary coils 25 and 26, respectively, which are coiled directly around magnetic cores; high withstand-voltage insulation layers which have a withstand voltage substantially in the range of from 6000 to 7000V are interposed between the secondary coils 16, 17 and the tertiary coils 25, 26, respectively, to provide therebetween sufficient electrical insulation and tight magnetic coupling.
The tertiary coils 25 and 26 are interconnected at one end in opposite phase so that their induced voltages cancel each other. The other ends of the tertiary coils 25 and 26 are connected to the input side of a rectifying smoothing circuit 27. The output side of the circuit 27 is connected via a Zener diode 28 to both ends of a parallel circuit of a resistor 31 and a capacitor 32, and the both ends are connected to the gate and cathode of a triac 33. The triac 33 is connected between the input terminals 14 and 15 via a relay 34, and a relay contact of the relay 34 forms the switch 13.
Under normal operating conditions, the voltages that are induced in the tertiary coils 25 and 26 are nearly equal but opposite in phase, and consequently, the input voltage to the rectifying smoothing circuit 27 is substantially zero. However, a ground fault of the lamp 23 or its wiring develops a short circuit across that one of the secondary coil which is grounded, and the induced voltage of the tertiary coil coupled with the grounded secondary coil significantly decreases, leading to the application of the entire induced voltage of the other tertiary coil to the rectifying smoothing circuit 27. This voltage is rectified and smoothed, and the output voltage rises, turning ON the Zener diode 28. As the result of this, the triac 33 turns ON to actuate the relay 34 and hence open the switch 13, shutting off the supply of the input AC power to the transformer 11. The relay contact of the switch 13 is connected to a normally open side NO, through which an operating current passes to the relay 34.
As is schematically shown in FIG. 2 wherein the parts corresponding to those in FIG. 1 are identified by the same reference numerals, there may be cases where the case 36 of the leakage transformer 11 is not grounded and hence the connection point C of the secondary coils 16 and 17 is not grounded either (but connected to the case 36) and the intermediate point between adjacent gas-filled lamp 23 is grounded. In such a situation, since the connection point C of the secondary coils 16 and 17 is isolated from ground although the wiring of the afore-mentioned one of them or the lamp 23 is grounded, the induced voltages of the secondary coils 16 and 17 and consequently the induced voltages of the tertiary coils 25 and 26 are not determined; therefore, no change is detected in the difference between the induced voltages of the tertiary coils 25 and 26. In other words, no ground fault protection is provided.
This will be further described below. When the lamps 23 are all being lit under normal conditions, the potential at the connection point C of the secondary coils 16 and 17 with respect to a ground E is determined by impedance values of loads including the lamps on both sides of the ground E, and varies with the state of equilibrium between the impedances of the both loads, and hence it does not become constant. Accordingly, the voltages that are induced in the tertiary coils 25 and 26 in accordance with the voltages of the secondary coils 16 and 17 do not become stabilized, and do not always become equal to each other.
Now, assume that a ground fault occurs at the point A on the side of the secondary coil 16 in FIG. 2. The potential at the connection point C of the secondary coils 16 and 17 with respect to the ground E depends on a load potential at the point B and the potential of the secondary coil 16 on the point A side; this potential does not become constant, either, but it becomes lower than the potential under the normal operating conditions. The same is true of the case where a ground fault occurs only at the point B. When the point A or B is grounded, however, substantially no difference arises between the voltages that are induced in the tertiary coils 25 and 26 in accordance with the induced voltages of the secondary coils 16 and 17. On this account, the voltage difference cannot be distinguished from the voltage difference between the tertiary coils 25 and 26 based on an imbalance between the load impedances.
Moreover, no circuit has been used to detect and protect against no-load conditions such as disconnection of the wiring of the lamp 23 or cracking of its lamp envelope. In addition, high dielectric-strength insulating layers, which withstand high voltages substantially in the range of from 6000 to 7000V at all times, have been interposed between the secondary coils 16, 17 and the tertiary coils 25, 26, respectively--this has inevitably raised the manufacturing costs.